In a radio communication apparatus for switching transmission and reception in time division, it is necessary to switch the connection between an antenna and a transmission/reception circuit. Alternatively, in a terminal using a plurality of frequency bands, a plurality of transmission/reception circuits are generally incorporated for the respective frequency bands, and hence it is necessary to switch signal paths between an antenna and the transmission/reception circuits. Alternatively, in a radio communication apparatus adopting diversity reception or MIMO (Multi Input Multi Output) systems, it is necessary to switch signal paths between a plurality of antennas and transmission/reception circuits
A high frequency switch circuit is used for switching the signal paths. A high frequency circuit using a depletion type high electron-mobility transistor (HEMT: High Electron-Mobility Transistor) as a switch has excellent characteristics, and hence is widely used. Generally, the depletion type high electron-mobility transistor (HEMT) normally exhibits a depletion type characteristic that when the gate potential is equal to the drain/source potential, the transistor is brought into an on-state where the drain and the source are connected to each other by a low resistance, and that when the gate voltage is lower than the drain/source voltage by a threshold voltage (about 1 V), the transistor is brought into an off-state where the drain and the source are connected to each other by a high impedance.
Therefore, in the case where a high frequency switch circuit is integrated, it is more advantageous in terms of production to use the depletion type field-effect transistor for all transistors in the circuit. In order to constitute a high frequency switch circuit by using the depletion type field-effect transistor without using a negative power source, there is proposed, for example, a constitution as described in Japanese Patent Laid-Open No. 9-98078 (Patent Document 1). Further, as another method, there is proposed a circuit for generating a control voltage by using the depletion type field-effect transistor, in Japanese Patent Laid-Open No. 2000-004149 (Patent Document 2).
In addition, it is desirable that a voltage sufficiently higher than the above described threshold voltage is applied as the control voltage, but the control voltage tends to be lowered according to a trend toward a lower voltage in a recent radio communication apparatus. For example, in a mobile radio terminal as represented by a portable telephone, since a battery is used as a power source, it is strongly required to reduce power consumption in order to extend the operating time of the terminal. A digital logic circuit is used for performing various control and modulation/demodulation in the radio terminal. In order to suppress power consumption in the logic circuit, it is effective to design so that the operating voltage of the logic circuit is suppressed to be low. Therefore, in the case where the high frequency switch circuit is directly controlled by the logic circuit, it is desirable that the control voltage necessary for the high frequency switch circuit is set to be low.
However, the electric power which can generally be interrupted by an HEMT transistor is proportional to the square of the difference between the gate potential and the drain/source potential. Thus, if the control voltage is lowered, the potential difference is also reduced, as a result of which the electric power which can be handled by the high speed switch circuit is also reduced. In order to avoid this, there is proposed a method in Japanese Patent Laid-Open No. 10-84267 (Patent Document 3) and Japanese Patent Laid-Open No. 2004-48692 (Patent Document 4), in which a high frequency signal is rectified and added to a control signal, so as to increase an internal control signal.
However, in the high frequency switch circuit disclosed in Patent Document 1, when the change in the control voltage is not sufficiently large as compared with the absolute value of pinch-off voltage of the depletion type field-effect transistor used in the circuit, the circuit becomes unable to function as the switch. This problem will be described in detail below.
FIG. 1 shows a high frequency switch circuit described in Patent Document 1. In the figure, reference numerals 6 and 7 denote depletion type field-effect transistors. Reference characters 11a and 11b denote external-control-signal-input terminals. Reference numerals 14 and 15 denote an input terminal 14 and an output terminal 15 of a high frequency signal, respectively. Reference characters 21a and 21b denote resistors. Reference characters 24a, 24b and 24c denote capacitances (capacitors), respectively.
The control is performed by applying a differential voltage between the terminals 11a and 11b. At this time, it is assumed that the operating voltage of the logic circuit is set to 1.6 V, and the pinch-off voltage of the depletion type field-effect transistor used in the high frequency switch circuit is set to −1.5 V. It is also assumed that a High voltage (1.6 V) is applied to the terminal 11a and a Low voltage (0 V) is applied to terminal 11b, so as to bring the high frequency switch circuit into the off-state. At this time, the drain and source voltages of each of the field-effect transistors (6, 7) are both boosted to 1.6V by the forward current of the Schottky junction of the field-effect transistor 6. Therefore, the gate-source voltage of the field-effect transistor 7 is set only to −1.6 V.
For this reason, when a high frequency signal causing the source-drain voltage of the field-effect transistor 7 to be changed by about 0.2 V is input into the field-effect transistor 7, the field-effect transistor is brought into the on-state, so that the high frequency switch circuit becomes unable to secure sufficient isolation. This results in a problem that the withstand power characteristic is reduced.
The above described problem of the withstand power characteristic can be solved by bringing the pinch-off voltage close to 0 V. However, when the pinch-off voltage is brought close to 0 V, the on-resistance of the field-effect transistor is increased, so that the insertion loss of the high frequency switch circuit is increased. For the above described reason, in the high frequency switch circuit proposed in Patent Document 1, there is a high probability that a problem may arise when an output of a logic circuit whose voltage is lowered is used as it is for the control of the high frequency switch circuit.
Further, in the inverter circuit constitution described in Patent Document 2, there is a disadvantage that even when the output terminal of the inverter is in a Low state, a voltage twice the on-voltage of a diode is output so as to reduce the change in the output voltage. Even if a differential voltage is produced by using the inverter, when the power supply voltage is set to 3 V which is a voltage of a lithium ion battery currently used in a portable telephone, the on-voltage of the diode is about 0.7 V, and hence a positive voltage of 1.4 V is output even in the case of the Low voltage output. In consideration of that the output of the High voltage is at most 3 V of the power supply voltage, the obtainable voltage difference is only 1.6 V, and hence the off-state of the field-effect transistor cannot be sufficiently secured. This results in the same problem, as in the case of Patent Document 1, that the withstand power characteristic is reduced.
In the system proposed in Patent Document 3, a switch circuit for switching a control signal needs to be provided. This results in a problem that a power source needs to be separately provided for the switch circuit, or that the number of elements constituting the circuit is large.
Further, in Patent Document 4, there is a problem that in the booster circuit of any of the proposed systems (FIG. 11, FIG. 12, FIG. 13, FIG. 14 in Patent Document 4), even in the case where the external control voltage is set to be low, when a high-power high frequency signal is applied to a high-frequency-detection terminal, a potential due to the rectifying action is added so as to increase the output voltage, and thereby a sufficient signal isolation performance cannot be obtained.